1. Field of the Invention
The present invention relates to a charge transfer device and a method of driving the charge transfer device. For example, in an application of the present invention to a CCD solid image pickup device and an image pickup apparatus, an initialization voltage is applied at the time of starting or the impurity concentration in a region below each of channels of charge transfer sections is increased to quickly deplete the channels, thereby enabling correct transfer of electric charge in a short time after starting.
2. Description of the Related Art
Conventionally, a CCD solid image pickup device is arranged so that, in each of a horizontal charge transfer section and vertical charge transfer sections, potentials in a depleted channel are successively changed to transfer accumulated charge generated by photoelectric conversion sections.
FIG. 4 is a plan view of a CCD solid image pickup device of this kind. A CCD solid image pickup device 1 has photoelectric conversion sections 2 formed of photosensors and disposed in a matrix form, and also has vertical transfer sections 3 and a horizontal transfer section 4. The vertical transfer sections 3 are disposed between columns of the photoelectric conversion sections 2 arranged in the horizontal direction. The horizontal transfer section 4 is disposed at the lower ends of the vertical transfer sections 3. Each photoelectric conversion section 2 accumulates charge by photoelectric conversion of incident light. Each vertical transfer section 3 reads out accumulated charge in each photoelectric conversion section 2 in a certain cycle and successively transfer the read-out accumulated charge to the horizontal transfer section 4. The horizontal transfer section 4 successively transfers to a charge detection section 6 the accumulated charge transferred from the vertical transfer sections 3. The charge detection section 6 converts the accumulated charge into an electrical signal and outputs the electrical signal.
The vertical transfer sections 3 and the horizontal transfer section 4 are formed in such a manner that transfer electrodes formed of polysilicon are arranged in an overlapping manner over channels of a semiconductor substrate formed by a predetermined process, with an insulating film interposed between the transfer electrodes and the substrate. In the vertical transfer sections 3 thus formed, four-phase drive pulses, for example, are applied to successively to change potentials below adjacent pairs of the electrodes so that accumulated charge is transferred. On the other hand, the horizontal transfer section 4 is formed so that the impurity concentration is changed in the channel below each of adjacent pairs of the electrodes, so that the adjacent pairs of the electrodes form electrode pairs. Two-phase drive pulses are applied to each adjacent electrode pair to successively transfer accumulated charge.
In a CCD solid image pickup device of this kind, the channels of the horizontal transfer section and the vertical transfer sections are full of charge when the operation is stopped. Therefore, it is difficult to output correct accumulated charge at the time of starting unless the channels are depleted by discharging the channels of the charge filling the channels. Ordinarily, this discharge is made by a reset drain of the charge detection section via the horizontal transfer section, as in the case of outputting accumulated charge.
In the conventional CCD solid image pickup device, therefore, a substantially long time, corresponding to several fields, is required for depletion of the channels immediately after a moment at which application of drive pulses is started by turning on power supply, and there is a problem of difficulty in enabling the device to correctly output accumulated charge in a short time after starting.